1. Field of the Invention
The present invention relates to an image reading apparatus for reading book-like documents from an overhead direction, and more specifically relates to an image reading apparatus which measures the distance to the document surface, and focuses and corrects for image distortion during image reading.
2. Description of the Related Art
An image reading apparatus for reading documents having a spatial curvature such as a book are known which, for example, detect a height of a surface of a document disposed face up on a document table, and scan the document image while focusing based on said detection. The aforesaid image reading apparatus focuses by adjusting the distance between the document surface and a unit by moving said unit in the direction of the optical axis of a lens, said unit comprising a line sensor such as a charge-coupled device (CCD) which accomplishes photoelectric conversion, the lens which forms the document image on said line sensor. As the focusing operation occurs, the document is read simultaneously by a main scanning accomplished by the line sensor in the unit, and a subscanning accomplished by moving the document in a subscanning direction (direction perpendicular to the main scanning direction of the line sensor).
Another image reading apparatus are also known, wherein after the document image data are read by the scans in both the main scanning and subscanning directions via an area reader, the document image data distorted by compression such as the binding area and the like is corrected based on document height detection, i.e., the image distortion correction is accomplished.
In the former image reading apparatus, reading of the document in the subscanning direction must be accomplished on the optical axis of the lens (namely, line sensor must be fixedly positioned on the optical axis prolongation line), which is disadvantageous inasmuch as there is a large amount of movement of the document moving portion during subscanning, such that the apparatus cannot be rendered in compact form.
In the latter image reading apparatus, however, reading of the document in the subscanning direction is possible off the lens optical axis, such that there is not any movement of the document moving portion for subscanning. Since the lens only moves along the optical axis, however, the line sensor (reading portion) reads the image from a position above the lens, such that the off-axial position of the lens causes the document to be read in an inclined direction. When the height of the document changes, reading position of the document and the height data detection position for correcting read image data are different. Furthermore, when the lens alone is moved for focusing, the document reading position is moved at the off-axial position relative to the optical axis, such that focusing and correction of image distortion cannot be accurately corrected during image reading.